The present invention relates to methods for determining cloud heights within a three-dimensional space.
Safety is a primary concern in the aviation industry. Unanticipated or unmeasured weather conditions can pose a threat to flight safety; accurate and appropriate weather information, particularly at airports, is therefore very important to pilots and to the air traffic control system. Visibility and ceiling information are of primary importance.
Ceiling is defined as the height above the ground from which prominent objects on the ground can be seen and identified or as the height above the ground of the base of the lowest layer of clouds when over half of the sky is obscured.
Ceilometers are devices designed to measure cloud height generally along a single dimension vertical line above the instrument. Common units use eye-safe pulsed diode lasers operating as LIDAR (light detection and ranging) devices. The outgoing laser beam is scattered by water droplets, and the backscattered beam is detected and analyzed by the ceilometer to determine characteristics of the scatterers (i.e., opaque or translucent clouds) and the range (derived from the transit time of the light beam).
The National Weather Service has installed Automated Surface Observing Systems (ASOS) at 900 airports. The ASOS system measure winds, temperature, dew point, pressure, visibility, rain, and ceiling or cloud height as a stand-alone automated system. This system provides weather data in METAR code for meteorologists and others within the aviation industry.
A significant limitation of the ASOS system is that cloud height data is one-dimensional because cloud height information is only measured along a single vertical path above the instrument. The standard instrument used in this application is the Vaisala CT-12K Light Detection and Ranging (LIDAR) ceilometer, which has a range of approximately 12,000 feet. Currently, the National Weather Service is upgrading from the CT-12K to the CT-25K ceilometer, which has a range of approximately 25,000 feet.
As part of this upgrade, the National Weather Service is interested in providing more information on ceilings and in providing data acquisition that will more adequately describe cloud heights over a larger area. Improving this technology to provide more information is a continuing effort.
The invention provides ceiling information in three dimensions, which covers all areas of a major airport, and which provides significant improvements over existing systems.
The present invention provides a method of 3-D measurement and visualization of cloud formations using a ceilometer and computer software. Data collected from the ceilometer contains cloud range information in polar coordinates (r,xcex8,xcfx80) which is then converted to a cloud height depiction in a 3-D space. Visualization of data is enhanced by animating consecutive sets of data in a time loop to show the movement and evolution of the clouds over the measurement site during an extended period of time.
The omni-directional platform described herein includes physical hardware and motion control computer hardware and software. The invention is produced for potential application to both the Vaisala CT-25K and the CT-75K ceilometers, as well as ceilometers from other manufacturers. The CT-75K has an approximate range of 75,000 feet and utilizes an array of four CT-25K ceilometers housed in a single enclosure and operating in parallel.
Pointing hardware can be either a combination of servo motors and gears that physically control the orientation of the ceilometer, or a scanning mirror configuration where servo motors control a gimballed mirror that reflects the ceilometer output and return at specified angles.
The data acquisition and control system of the present invention controls the motion of the ceilometer or mirror and acquires data from the units. Running on a personal computer or on an embedded system, motion control and data acquisition routines command motion control boards and data acquisition boards connected to the computer""s motherboard.
Conventional use of the CT-25K system for measurement of cloud height produces only a single reading at any instant in time. The present invention provides utilization of the instrument in a scanning mode and provides processing and display of the data in three dimensions. The Interactive Data Language (IDL(copyright)) developed by Research Systems, Inc. (RSI) was selected for this purpose. IDL has an array-oriented architecture specially developed for handling large amounts of complex data and has been widely used in a variety of applications including meteorology, astronomy, and fluid dynamics. IDL""s ability to manipulate and display three-dimensional images is especially notable.
For 3-D displays of the cloud hit data, it is necessary to create a 3-D data-element representation that reflects the spatial resolution of the measurement. Cloud ceiling height measurements are performed with constant elevational and azimuthal increments.
Once the 3-D model is created, it is manipulated to produce views from anywhere in the field, or to provide xe2x80x9cfly-byxe2x80x9d simulations. Additionally, multiple data sets are incorporated into a movie format for temporal data visualization.
Elevation angles are measured with a mechanical inclinometer, with a measurement precision of +/xe2x88x920.5 degrees. Azimuth angles are determined from compass measurements, pointing markers or sensors on the pedestal base and fixed reference indicators.
The present invention employs a new version of the Vaisala system, the CT-75k, which has a range of 75,000 feet. Coupling this device with an Az-El (azimuth/elevation) scanning system allows probing the atmosphere in three dimensions around the device location to a range of nearly 15 miles. The data output from the ceilometer is processed by a computer along with elevation and azimuth angles from shaft encoders on the scanning mechanism to produce a true three dimensional set of data for the scanned volume.
Scanning methods of the Omni-Directional Cloud Height Indicator include pointing the entire LIDAR unit or employing rotating mirrors to deflect the laser beam throughout the probed volume.
The rotating mirror version utilizes a LIDAR ceilometer that is permanently mounted to a pedestal with the output window facing below the horizon. A two-axis pan/tilt scanning mirror system is mounted to the output end of the ceilometer. The pan/tilt mirror is mechanically rotated in two axes using a pair of stepper motors. Accurate microstepping is accomplished using a stepper-motor controller/driver electronics card with 16-bit precision. The micro-stepping feature enables 10-arcminute incremental positioning of the pan/tilt mirror. Each step corresponds to 10-arcminutes of mirror motion and therefore a 20-arcminute angular deviation of the optical axis. Mirror movement provides for 170 degrees of deflection, of the optical axis, in one axis and 110 degrees in the second axis.
A three-dimensional, cloud height model is generated by scanning through a solid angle using the mirror pan/tilt scanning system. At each location in the scan matrix, cloud height information is calculated by the ceilometer. The scan mirror contains a highly reflective durable coating in accordance with Military-Specification Mil-C-48497. The coated mirror surface defects shall not exceed a scratch-dig value of 80-50 and the surface figure shall not contain errors that exceed 0.001 inch/inch.
The scanning mirror system is lightweight, low cost and eliminates the need to scan the large mass of the ceilometer system. An enclosure housing covers the mirror scanning mechanism and contains an optical window that transmits the laser beam. The enclosure protects the system and all moving parts from being exposed to adverse weather conditions.
Besides the scanning function, the platform determines the pointing angles, both elevation and azimuth, of the LIDAR beam. Devices such as shaft encoders are employed on both axes to provide the required pointing information. Pointing information is sent to the computer along with the LIDAR backscatter data.
To obtain the maximum range of the LIDAR, the dwell time per unit data is 15 seconds. Depending upon the density of data points desired (and thus the size of the angular steps in both elevation and azimuth), a scan of the hemisphere requires a significant amount of time (i.e., up to a few tens of minutes for 8 points per azimuth scan and 10 elevation angles). Thus, the data output for a complete scan of the hemispheric volume may be limited to several sets per hour. Alternatively, dwell time is reduced, or instruments are duplicated.
The new ceilometer is capable of scanning in three dimensions to detect clouds within a range of 75,000 feet (about 15 miles), using a Vaisala CT-75k ceilometer, which has a nominal range of 75,000 feet. The ceilometer is operated on a scan and tilt platform to probe the atmosphere in a hemisphere above the ground plane. The cloud height (or range) data from the laser backscatter is analyzed and processed by a computer to convert range and angle measurements into a three dimensional plot of clouds within the probed volume.
The ceilometer is used on a scan and tilt platform also to detect low clouds and fog near the horizontal plane.
Available CT-25k ceilometers may be used. The CT-75k version increases the range of measurement from 25,000 feet (4.7 miles) to 75,000 feet (14.2 miles). The extended range plus the expansion of probed volume from a single dimension (fixed vertical beam) to the three dimension scan (probing the above-ground hemisphere) are major improvements over current cloud height measurements.
These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings.